There are cases in which two rotational members, such as concentrically arranged outer and inner rolls, are shifted in their rotational state as needed, from a synchronously rotating state in which they rotate synchronously with each other into a differentially rotating state in which they rotate differentially with a certain speed difference. There are also cases in which a rotational plate and a phase gear for rotating a table are rotated in the same direction and, when required, only one of the rotational plate and phase gear is rotated in order to provide a phase difference between them. Such operations are performed by a phase-angle providing rotational device disclosed in Patent Document 1, for example.